Field of the Invention
Embodiments of the present invention relate to radiation distribution systems, including optical systems that produce a batwing-type light distribution pattern.
Description of the Related Art
Many lighting applications require a fixture that produces a batwing-type distribution of light. The term “batwing” refers to a light distribution whose luminous intensity is greater along a direction at a significant angle relative to the main axis of distribution rather than along a direction parallel to the main axis. The desirability of a batwing distribution is evident in many lighting applications, including, for example, roadway lighting in which most of the light should be distributed in a direction parallel to the roadway.
FIG. 1a shows various types of roadway illumination patterns named with a convention used by the Illuminating Engineering Society (IES). As shown, there are five common types of roadway illumination. Type I illumination is a direct illumination in two directions along the direction of the roadway (if the road is a single road) and/or in a straight directional pattern at a cross section as shown by the Type I-4-Way pattern. Type V describes an omni-directional lighting pattern across the entire intersection. Type II is similar to Type I, but the light fixture is mounted above a point displaced away from the center of the region to be illuminated. Type III illumination shows a different angled illumination from normal as compared to Type II, where the angle of illumination from normal is narrower to reflect a smaller coverage area. Type IV illumination has an even narrower angle of illumination from normal to create a different, smaller illumination area than either Type III or Type II.
FIG. 1b shows a known generic light fixture 100 that is mounted at a height H above a surface 102 that is to be illuminated by the fixture 100. The main axis 104 starts at the fixture 100 and runs perpendicular to a plane containing surface 102. The light distribution on the surface is typically specified in terms of the illuminance I(x,y) measured in lumens/ft2. The distribution of the light emanating from the fixture is typically specified in terms of the luminous intensity P(φ,θ) measured in a direction making an angle φ relative to the main axis and lying in a plane that contains the main axis and is oriented at angle θ, as shown in FIG. 1b. The illuminance and the luminous intensity are related by:
                                          I            ⁡                          (                              x                ,                y                            )                                =                                                    P                ⁡                                  (                                      ϕ                    ,                    θ                                    )                                                            H                2                                      ⁢                                          cos                3                            ⁡                              (                ϕ                )                                                    ⁢                                  ⁢                  x          =                      H            ⁢                                                  ⁢            tan            ⁢                                                  ⁢            ϕ            ⁢                                                  ⁢            cos            ⁢                                                  ⁢            θ                          ⁢                                  ⁢                  y          =                      H            ⁢                                                  ⁢            tan            ⁢                                                  ⁢            ϕ            ⁢                                                  ⁢            sin            ⁢                                                  ⁢            θ                                              (                  Eq          .                                          ⁢          1                )            where H is in feet.
In many applications, it is desirable to illuminate a region of a surface that is approximately rectangular or elliptical in shape. In FIG. 1b, the elliptical region 106 is substantially longer in one direction (along the x-axis) than in the other direction (along the y-axis). Outside the region 106 the illuminance falls off to a minimum level. In some applications (for example Types I, II, III, and IV from FIG. 1a), the region 106 may be substantially symmetrical about the main axis in the x-direction of the pattern and either symmetrical (Type I) or asymmetrical (Types II, III, IV) along the y-direction. The illuminance distribution can be approximately characterized by the illuminance vs. position functions along the axes, I(x,0) and I(0,y), and the luminous intensity along the two axes is:
                                                        P              x                        ⁡                          (              ϕ              )                                =                                    H              2                        ⁢                                          I                ⁡                                  (                                      x                    ,                    0                                    )                                                                              cos                  3                                ⁡                                  (                  ϕ                  )                                                                    ⁢                                  ⁢                                            P              y                        ⁡                          (              ϕ              )                                =                                    H              2                        ⁢                                          I                ⁡                                  (                                      0                    ,                    y                                    )                                                                              cos                  3                                ⁡                                  (                  ϕ                  )                                                                                        (                  Eq          .                                          ⁢          2                )            where H is the height (in feet) of the source above a surface.
For applications where the maximum value of φ is 20-25° or less, the 1/cos3(φ) factor is less than about 1.3, and many such lighting applications can use conventional collimators to achieve acceptable uniformity. For other applications, however, the illuminance is desired to be uniform out to values of φ of 30° or more. The 1/cos3(φ) factor at these angles rises sharply with φ, reaching values much greater than 1 well before the illuminance factor drops off. The characteristic batwing shape of P(φ) is critical in these applications in order to achieve substantial illuminance uniformity.
FIG. 2 shows a pair of graphs showing distributions for an exemplary roadway luminaire mounted at H=25 ft, calculated according to the given equations. FIG. 2 shows a uniform illuminance I(x,0) denoted as 204, extending ±50 ft in the x-direction of FIG. 1, and a uniform illuminance I(0,y) denoted as 208, extending −10 to +25 ft in the y-direction, with each illuminance falling off gradually outside those ranges. FIG. 2 also shows the corresponding intensity distributions 202 and 206, both batwing distributions. The batwing distribution 202 is along the long x-axis of the region 106 (e.g., along the roadway in the street lighting application), and the batwing distribution 206 is along the perpendicular y-axis.
Batwing luminaires are known in the prior art for use with incandescent and discharge lamps which are typically small sources emitting into a full spherical distribution. These sources are typically powerful enough such that one or two lamps can supply all the light needed for the entire fixture. Batwing optical systems for these sources typically use reflectors having asymmetric curvature, facets, or cut-off angles, as can be found in the prior art. Lenses are less common, but are also known, particularly Fresnel lenses or lenticular lenses. These prior art systems are adapted for incandescent and discharge lamps.
Recently, light-emitting diodes (LEDs) have become common in many lighting applications. The batwing optical systems used for incandescent and discharge lamps are not designed for use with LEDs. LEDs are typically arranged in arrays that have a large overall area. LEDs also emit only into the forward hemisphere, not into a full spherical pattern. It is desirable to have an improved batwing optical system adapted for light sources with these emission characteristics, and specifically for LEDs.